The present invention relates to two dimensional profiling using a contact force atomic force microscope (AFM) and specifically relates to profiling sidewalls of lines and trenches in integrated circuits.
There are commercially available atomic force microscopes (AFM) which operate in a contact mode, that is, the tip and the sample are in contact while the tip scans the surface of the sample. One such apparatus is described in the article entitled "Using the Atomic Force Microscope to Measure Submicron Dimensions of Integrated Circuit Devices and Processes" by M. P. Rodgers and K. M. Monahan, SPIE, Vol. 1464 Integrated Circuit Metrology, Inspection and Process Control V, 1991, pages 358-366.
A major advantage of contact mode atomic force microscopes is the simplicity of construction. A major disadvantage of these atomic force microscopes is the inability of the AFM to profile high aspect ratio structures having steep sidewalls. Contact mode AFMs are unable to readily accommodate long and narrow-angle tips or "boot-shaped" tips because of the simple detection system used to measure the tip motion.
The present invention overcomes these disadvantages of two-dimensional profiling of a surface using a contact mode AFM, particularly for profiling sidewalls of steep lines and trenches of integrated circuits. The present invention relies upon the two-dimensional detection of the tip motion for controlling the scanning of the tip over the sample surface. The invention also concerns the optimization of the cantilever and tip dimensions of an atomic force microscope.
Detection of tip motion in one dimension involves the use of tunneling, capacitive, inductive or optical methods. These methods can be modified to permit the measurement of the tip motion in two dimensions.
Two dimensional detection of the tip motion is described in the article entitled "Simultaneous Measurement of Lateral and Normal Forces with an Optical-Beam-Deflection Atomic Force Microscope" by G. Meyer and N. M. Amer, Appl. Phys. Lett. 57 (20), Nov. 12, 1990, pages 2089 to 2091 which is incorporated herein by reference. In the described apparatus, the tip is attached to a spring-like cantilever. Vertical tip motion causes the cantilever to bend in a conventional up/down manner. Lateral tip motion causes the cantilever to twist along its longitudinal axis. A laser beam deflected from the cantilever is detected by a 4-quadrant detector. The vertical tip motion and lateral tip motion is measured from the four detected signals.
Another two dimensional detection arrangement involves the use of piezo-resistive measurement of the cantilever bend. In this arrangement, the cantilever is made of partly doped semiconductor material, such as silicon. The strain induced in the cantilever when the cantilever is bent causes a resultant change in the resistance of the semiconductor material which is then measured. The cantilever is bifurcated in order to provide two independent resistance signals for measuring two dimensional motion of the cantilever. Both bifurcated regions comprise doped semiconductor material regions. Electrical contacts are provided at the respective distal ends of each region and at the center of the cantilever where the regions are joined. These three contacts form two resistor circuits, i.e. each respective region with the center, which are inserted into a bridge circuit. The difference between the two resistances is measured by the bridge. Additionally, the combined series resistance of the two regions is a measure of the overall bending of the cantilever in the vertical direction.
The profiling of vertical surfaces in a substrate using non-contact scanning force microscopy is described in co-pending patent application Ser. No. 07/830,804 assigned to the same assignee as the present invention, which is incorporated herein by reference.